The way I use and create monsters in my Swords and Wizardry game is different than many game masters. My primary use is as metaphors for the themes I am working into the campaign. They can be used to create emotional responses such as fear, horror, disgust and anxiety.
Monsters are usually a novel tactical puzzle with a veneer of story covering up the mechanisms. This is not a bad thing. Kicking in doors and slaying whatever eldritch horror is occupying the chamber can be a lot of fun.
I take a lot of inspiration from Tolkien. His monsters were metaphors for the worst sins of humanity. Shelob wasn’t merely a giant sentient spider. She was a demon in spider form. She didn’t control insects. She wove death and hated the light. Ungoliant consumed the trees of the Valar, destroying the light of their creation. The spiders of Mirkwood made the forest dark and dangerous. The land around the Lonely Mountain was desolation. The only things living were small creatures beneath the notice of Smaug. Tolkien’s monsters are ecological disasters. Mordor is the mother of all Superfund sites.
Monsters are like an extreme form of invasive species. Monsters aren’t creatures like a beetle that bores into trees that have no defense against the novel pest or knotweed choking out all the other plants. Invasive species alter their new habitat or dominate it when they are introduced. The difference is that an invasive species is outcompeting other creatures in its niche and a monster is destroying everything it can.
Monsters are beings animated by the spirit of Chaos. There is some Law in them, otherwise they could not take on a persistent form. They are not life with a balance of Law and Chaos. They are imbalanced. More Chaos than Law with just enough Law that the forces of Chaos cannot control monsters. Chaos can intimidate or frighten monsters into servitude, for a while.
Even Chaos hates monsters.
Monsters are bent on destruction for their own ends. Whoever or whatever gets in the way is consumed and discarded. For the monster, redemption is not possible. For the monster, reform is unavailable. They want to destroy and that is all.
You can destroy a monster. You can drive it off. You can chain it or cage it. Maybe, you can neutralize it.
If you try to make it your friend, or use it for your own purposes, you’ll suffer. You might become a monster yourself.
People, can choose to behave monstrously. Most of us do from time to time. We all have the shadow within us. People choose to cause suffering. The Chaos within us pushes us toward destruction. Yet, we have a choice. Even a person who chooses to behave like a monster can be redeemed, or rehabilitated, if they choose it.
If they become a monster, there is no redemption.
Monsters are unnatural. They don’t reproduce in the way animals do. Many monsters spontaneously generate. Goblins generate from improperly disposed of refuse. Many monsters are animals or people who have been transformed by Chaos into a destructive horrific being.
People create many monsters. People can create monsters by accident through carelessness, arrogance, ignorance, and stupidity. Goblins are an example of people creating monsters through carelessness.
People can create monsters on purpose. Usually they justify their creations with resentment. That other man stolen my only true love. Those people aren’t giving me the respect I deserve. I’ll show them. I will get my revenge.
These cases are tragic. The monsters created by people were often once people themselves. Once they step over the threshold of becoming a monster, there is no coming back. Sometimes you can kill a person by making them a monster. Sometimes, people kill themselves by becoming a monster.
Then there are the Outsiders. These are beings from another dimension/multiverse/reality that feed on life in the prime material plane. Even the gods fear them.